The transmission of energy to the system of nuclear spins that represents the sample of material during the period of excitation is accomplished by means of the probe that carries the aforesaid sample.
The recovery of energy emitted by the system of spins returning to equilibrium is also accomplished by the probe.
The probe constitutes the essential part of a resonator. The spectrometric measurement of resonance is accomplished by it.
The probe is composed of a mechanical sample carrier component and of the antenna network electronic circuit that forms the radio resonator, or properly speaking, the microwave resonator. This network creates an electromagnetic wave in the range of radio or microwave frequencies concentrated in the volume containing the sample.
To make a spectrometric resonance measurement at very high temperatures using a resonator used in magnetic resonance, two essential conditions must be satisfied simultaneously:
the sample must be contained in the volume of radio frequency or microwave frequency irradiation,
the sample must be heated in the same irradiation volume to permit measurements at the desired temperatures.
At the present time, the sample is heated by a temperature-controlled stream of air passing through the chamber of the resonator enclosing the sample carrier.
The temperature is regulated at a set value, for example +100.degree. C., printed on the control panel, and maintained in the measurement chamber using a control loop, for example a thermocouple control loop.
For measurement temperatures between 200 and 600.degree. C., a special shielded probe is presently used in magnetic resonance measurements which is cooled by a pump and heat-removing fluid (usually oil).
This type of cooling necessary to protect the resonator adds greatly to the total cost of the measurement probe.
The radio frequency or microwave frequency emission by the sample is detected by a sensor sensitive in the radio frequency or microwave frequency band being studied.
Traditionally, this sensor is in direct contact with the sample.
Since the sample and sensor are necessarily joined, the temperature increase of the sample results in a corresponding increase of the thermal noise of the sensor, which masks the signal to be measured.
Furthermore, any thermal insulation by wrapping or otherwise is unsuitable because of the difficulties of the wrapper in maintaining the temperature, and for various other types of thermal insulation, because of their screening characteristics for radio frequency or microwave frequency waves and because of the noise signals that they may generate.
The second important condition for the measurement to have meaning is not to change the sample and to avoid any chemical reaction with the container that holds it.
Actually, the heated samples show physical and chemical characteristics themselves that may modify the characteristics of the sensor by reaction with the sample carrier.
Because of this, these temperature increases may induce chemical reactions by interaction with the support in the midst of the sample, whose nature is variable and thus significantly falsifies the measurements.
Finally, a significant elevation of the measurement temperatures is propagated to the resonator by thermal dissipation and thus modifies greatly its operating characteristics.